1. Field of the Invention
The present invention relates to a high-precision optical measuring apparatus arranged in, e.g., an environmental chamber. More particularly, the present invention is suitably applied to, e.g., a laser interferometric measuring device for measuring the coordinate position of a stage which is aligned with high precision in, e.g., a semiconductor exposure apparatus, a reticle coordinate measuring machine, or the like arranged in an environmental chamber.
2. Related Background Art
For example, in the manufacture of a semiconductor element by a photolithography process, an exposure apparatus for exposing a pattern image on a photomask or reticle (to be generally referred to as a "reticle" hereinafter) onto a wafer coated with a photoresist is used. In such an exposure apparatus, since the wafer must be aligned with high precision, the coordinate position of a stage for aligning the wafer is measured by a laser interferometric measuring device. The precision of a pattern formed on the reticle influences the yield of semiconductor elements formed by transferring this pattern. For this reason, a reticle coordinate measuring machine for measuring the pattern on the reticle is used. In the reticle coordinate measuring machine as well, the coordinate position of a stage for moving the reticle is measured with high precision by a laser interferometric measuring device.
Normally, such a semiconductor exposure apparatus, a reticle coordinate measuring machine, or the like is arranged in an environmental chamber connected to a high-precision air-conditioning apparatus. The following description will be made while using a semiconductor exposure apparatus as an example. The air-conditioning apparatus for the environmental chamber in which the semiconductor exposure apparatus is arranged has three main functions. The first function is to maintain cleanliness of the entire semiconductor exposure apparatus at a predetermined level, and the second function is to maintain the apparatus at a constant temperature. The third function is to suppress a variation in refractive index of the air which influences the precision of the above-mentioned laser interferometric measuring device and other optical sensors. The present invention mainly relates to this third function.
In the laser interferometric measuring device, a variation in refractive index of the optical path of a measurement beam is an error factor. More specifically, when the refractive index of the air varies due to a variation in temperature of the air, the length-measurement precision suffers. The rate of change in refractive index of the air with respect to a unit change in temperature is about -1 ppm/.degree. C. In this case, when the distance to be measured is assumed to be about 300 mm, a measurement error upon a variation in refractive index of the air is about 0.3 .mu.m per 1.degree. C.
For example, in the case of the next-generation semiconductor exposure apparatus which can manufacture 64M DRAMs, the measurement precision required for a laser interferometric measuring device for a wafer stage is about 0.01 .mu.m. For this reason, the temperature fluctuation of the air must be suppressed to be about 0.03.degree. C. or less. In order to reduce the temperature fluctuation of the air, the prior art pays attention to the performance of the air-conditioning apparatus for the environmental chamber, i.e., the temperature control performance of the air-conditioning apparatus is improved, and temperature nonuniformities at a supply opening of the air, from the air-conditioning apparatus, in the environmental chamber are reduced. In the case of the semiconductor exposure apparatus, an anti-dust HEPA filter (High Efficiency Particulate Air Filter) is arranged at a blowing opening of the environmental chamber.
In the above-mentioned prier art, especially in the semiconductor exposure apparatus, the HEPA filter is arranged at the blowing opening of the environmental chamber. For this reason, the fluctuation of the refractive index of the air itself immediately after the air is supplied from the blowing opening into the environmental chamber is very small. However, the fluctuation of the refractive index of the air gradually increases before the air supplied via the HEPA filter at the blowing opening reaches the optical path of the laser interferometric measuring device. As a result, a considerable variation in refractive index occurs in the optical path of the laser interferometric measuring device, resulting in poor measurement precision.
In this case, the dominant variation factor of the refractive index is temperature. Causes of a variation in temperature of the air include generation of a temperature fluctuation in a boundary layer, and diffusion of heat by convection transportation. More specifically, since the temperatures of metal members in the semiconductor exposure apparatus become slightly higher than the environmental air under the influence of various heat sources, a temperature gradient is formed in a boundary layer with an air layer (flow field) flowing around the metal members. Furthermore, since the air layer (flow field) is in a turbulent state, a temperature fluctuation of the air occurs. The temperature fluctuation of the air produced in the boundary layer is diffused throughout the blown air by convection. For this reason, even in the optical path of the laser interferometric measuring device, a temperature fluctuation of the air, and a fluctuation of the refractive index occur.